polar@top.cis.syr.edu (Polar Humenn) (12/27/90)
exuse me if this has already been posted.. I didn't see it come through.
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I have a question about the SUN-4 Unix loader (ld).
I am writing an assembler for a certain research project of mine,
the details are unimportant here.
My problem is trying to make Internal symbol references when I construct
the "a.out" file so that "ld" understands them.
This is a very simplistic attempt. I use only symbols and 32 bit relocation
addresses, no fancy pc relative or address relative offsets and the hoard
of other options outlined in "man 5 a.out".
Let us say I have,
main: push_structure whammy
push_closure foo
execute fine
whammy: push_constant 12
push_constant 13
enter_structure 2
Let us say that all instructions and operands each take 1 word, 32 bits.
let me define a couple of functions for clarity
unsigned addto_nlist( name, type, other , desc, value )
This function adds a symbol to the nlist. and assigns the corresponding
structure fields. Only type and value are really important here.
(I think!) It returns the index into the symbol string.
void addto_reloc_text( addr, index, r_extern, type, addend )
This function adds relocation information. I only relocate by symbols
so the index is always the index of the symbol.
So, I assemble in this order
-- main : push_structure whammy
(void) addto_nlist( "main", N_TEXT | N_EXT, 0, 0, 0x00000000 )
idx = addto_nlist( "whammy", N_UNDF | N_EXT, 0, 0, 0)
addto_reloc_text( 0x00000004, idx, N_EXT, RELOC_32, 0 )
-- push_closure foo
idx = addto_nlist("foo", N_UNDF | N_EXT, 0, 0, 0)
addto_reloc_text( 0x0000000c, idx, N_EXT, RELOC_32, 0 )
-- execute fine
idx = addto_nlist("fine", N_UNDF | N_EXT, 0, 0, 0)
addto_reloc_text( 0x00000010, idx, N_EXT, RELOC_32, 0)
-- whammy: push_constant 12
(void) addto_nlist("whammy", N_TEXT | NEXT, 0, 0, 0x00000018 )
As you may be able to tell from this gobitly gook is that I end up
with an a.out file such that "nm" looks like:
00000000 T main
U whammy
U foo
U fine
00000018 T whammy
Then I run "ld -r" on the a.out file and the symbols get resolved,
and I end up with:
00000000 T main
U foo
U fine
00000018 T whammy
This is fine and great. Now, what I would like to do is make "whammy" an
internal (local) symbol that will disappear. The documentation states that
if I prefix the name with a 'L' it will dissapear after being resolved.
However, I tried this and I get error messages relating to not being able
to find the symbol. I have tried all sorts of combinations of specifying
and not specifying N_EXT in the nlist and relocation information. However,
no luck. It appears the only way to get these symbols to resolve is to
declare them all N_EXT, the way I should you above.
That is unless, I am not understanding something. I don't have the code
for the linker, so I can't analyze what it is doing internally.
Basically, I want the loader "ld -r" to resolve my predesignated local symbols
and then strip them from the exported nlist. It appears to me right now, that
the only way I can do this is to reslove them in my assembler first, which would
really suck since *I know* "ld" has much quicker and streamlined algorithms for
doing such things (many years of improvements). i.e. Much better than I
would care to spend the time to write myself.
Can anyone help me into understanding the loader?
Please email me directly.
Thanks!!!
-Polar polar@top.cis.syr.edu